Real time data showed levels of coarse pollution particles (PM 10) peak up to 19 times the national safe standard for 24 hours and levels of fine, respirable particles (PM 2.5) peak to more than 10 times the safe standard at some locations.

Even sulphur dioxide (SO2) level were high, at least twice the standard, during the peak hours (9pm to midnight) in many parts of the city, indicating the use of crackers with very high sulphur content. This, despite traders claiming a serious slowdown in cracker sales this year.

Delhi government's press statement on Thursday, however, masked these peaks as it gave a wide range and 24-hour average for all the pollutants. Even those were severely high. Delhi government said its efforts had led to a drop in air pollution as compared to last year's Diwali. "Appeals of the chief minister, deputy chief minister and environment minister along with the initiatives of MLAs to citizens of Delhi bore desired results," the statement said.

Monitoring agency SAFAR's initial analysis, in fact, showed a rise in pollution over last year. What was uncontested was the fact that favourable weather — a breeze on Wednesday night and Thursday morning — helped clear out pollutants to an extent.

According to the Met department at IGI airport, "there were winds of 5 to 15 kmph persisting overnight with support from high vertical mixing of air that helped in dispersing smog," resulting in a clear morning.

A statement issued by the Central Pollution Control Board (CPCB) also concluded "that there was a significant shift in wind direction on the festival day, which resulted in less humidity profile besides increase in wind speed from 1.9 m/sec to 3.4 m/sec." This, it said, "increased atmospheric mixing height to the level of 855 metres and resulted in easy dispersion of air pollutants".

Said Gufran Beig, project director, SAFAR: "As expected, levels were severe on Diwali day and on Thursday. This time Thursday has borne the brunt of Diwali pollution despite the congenial weather. According to our estimates air pollution has gone up this year. We will release a conclusive report on Friday."

SAFAR's data showed Delhi's 24 hour average for November 11 to be 258 micrograms per cubic metres, about 4.3 times the standard.

Centre for Science and Environment (CSE's) "exposure" monitoring using portable air quality devices indicated severe levels of pollution. "The levels of tiny particles had already increased seven times since October 1 this year. This left no room for additional pollution in the city, especially from crackers that not only push up pollution but also lace the air with deadly cancer-causing substances," said Anumita Roy Chowdhury, head of CSE's clean air campaign.

Delhi government's release showed a marginal improvement in air quality. The average concentration of oxides of nitrogen (NO2) for the last 24 hours ranged from 37 to 79 micrograms per cubic metres while the range in 2014 was 39 to 194 micrograms per cubic metres. The average concentration of SO2 on Wednesday was 26 to 64 micrograms per cubic metres as against 8 to 87 micrograms per cubic metres last year.

The highest PM 2.5 levels were recorded at R K Puram, where the average of 369 micrograms per cubic metres was about 6 times the national standard and 14 times the WHO standard. The lowest level was at Civil Lines — 184 micrograms per cubic metres. The situation was no different in other locations like Mandir Marg and Anand Vihar. At Anand Vihar, PM 10 levels had reached a whopping 2,000 micrograms per cubic metre, 20 times the safe standard.

But Delhi Pollution Control Committee (DPCC's) real time data that TOI tracked on Wednesday night and Thursday early morning showed a very rapid and severe build-up of pollutants between 8pm and midnight. At R K Puram, for instance, which pretty much represents the entire south Delhi, PM 10 levels started rising rapidly from 8pm to reach 1,320 micrograms per cubic metres by midnight and levels of fine, respirable pollution particles that have serious health impacts peaking to 734 micrograms per cubic metres, which is about 12 times the national safe standard for 24 hours.

While these were the peak levels and they are usually not compared with ambient air standards, experts said the peaks reflect the dramatic drop in air quality due to firecrackers.

The CSE team moved with their portable air pollution monitoring devices in different parts of the city - Paharganj, Rajendra Nagar and Karol Bagh in the west; Lajpat Nagar in the south; Mayur Vihar, Patparganj and Laxmi Nagar in the east; around Dhaula Kuan in southwest Delhi and Sita Ram Bazar in old Delhi.

Overall, people breathed an hourly average that was at least three to four times higher than the ambient monitoring recorded at the DPCC stations, according to CSE researchers. After 8.30pm, for instance, Mayur Vihar Phase I recorded one-hourly average level of PM 2.5 as high as 1,763 microgram per cubic metre. In Patparganj, PM 2.5 had peaked to 2,114.

CSE's analysis of DPCC's data revealed SO2 build up at certain locations was dramatic. "SO2 levels are not a problem in Delhi any more. During pre-Diwali days on November 6 and 7, the SO2 level was as low as 23 but on Diwali, the higher range in 24 hours reached 64 microgram per cubic metre — an increase of 2.6 times," it said.

SO2 levels peaked to 250 micrograms per cubic metres. Depending on the exposure, SO2 inhalation can lead to bronchoconstriction (inflammation, constriction of air ways) and aggravated asthma symptoms, while PM 2.5 pollutants are so tiny that they can enter the blood stream. The combination of these two becomes deadly, and many people complained of burning eyes and respiratory discomfort through Wednesday night.

Noise pollution at three out of five locations had breached the safe standard by 1.5 times on Diwali night.

Ashwani Kumar, environment secretary, Delhi said, "There has been definitely a positive impact from awareness and campaigns. We have to go by 24-hour average and not peaks. Let's not overlook the improvement."